1. Field of the Invention
The present invention relates to a position detecting device, which ejects a pressure fluid supplied from a pressure fluid supply source from a detection nozzle with respect to a detection surface of a workpiece, and detects a back pressure from the detection surface to thereby detect the position of the workpiece.
2. Description of the Related Art
Heretofore, when carrying out processing on a workpiece by a machine tool or the like, it has widely been practiced to use a back pressure type of position detecting device in order to confirm the position of the workpiece (see, for example, Japanese Laid-Open Patent Publication No. 06-114685, Japanese Laid-Open Patent Publication No. 10-332356, and Japanese Laid-Open Patent Publication No. 2000-141166).
FIG. 11 is an explanatory drawing showing in outline a conventional back pressure type of position detecting device 150.
In this case, a pressure fluid supply source 152 is connected via a passage 154 to a supply port 156 of the position detecting device 150. Further, a detection port 158 of the position detecting device 150 is connected via a passage 160 to a detection nozzle 164 provided in a table 162. Furthermore, a workpiece 166 is arranged in confronting relation to a reference surface 165 of the table 162. Further, a surface (detection surface) 168 of the workpiece 166, which confronts the reference surface 165 of the table 162, is separated from the reference surface 165 by a distance X.
In this situation, when the pressure fluid is supplied from the pressure fluid supply source 152 via the passage 154 to the supply port 156, the pressure fluid, which is introduced into the position detecting device 150, is constricted or throttled by an internal nozzle 170 and is delivered to the detection port 158. The delivered pressure fluid is supplied to the detection nozzle 164 from the detection port 158 via the passage 160, and the pressure fluid is ejected toward the detection surface 168 of the workpiece 166 from the detection nozzle 164.
A pressure sensor 172 is connected to a detection port 158 side of the internal nozzle 170. The pressure sensor 172 detects a pressure (back pressure) of the pressure fluid during the time that the pressure fluid is ejected toward the detection surface 168 from the detection nozzle 164.
FIG. 12 is a graph illustrating a relationship between the distance X and the back pressure P2 for each of different pressures (supply pressures) P1 at which the pressure fluid is supplied to the supply port 156. As shown in FIG. 12, the supply pressure P1 ranges from Pa to Pc, in which the relationship Pa>Pb>Pc is satisfied. In this case, the position detecting device 150, under a condition in which the workpiece 166 is arranged at a predetermined distance X0, detects the back pressure P2 at a time that the pressure fluid having a supply pressure P1 (initial set pressure) is supplied to the supply port 156 from the pressure fluid supply source 152, and the detected back pressure P2 is set at a threshold value of P0. Moreover, in FIG. 12, the back pressure, which corresponds to the distance X0 in the graph where P1=Pb, is set at the threshold value P0. Further, the distance X0 is defined as a maximum distance at which it can be regarded that the workpiece 166 is seated with respect to the table 162.
Next, in the position detecting device 150, when the back pressure P2 is detected by the pressure sensor 172 at the time that the pressure fluid is ejected toward the detection surface 168 of the workpiece 166, which is arranged at an arbitrary distance X from the detection nozzle 164, in the event that the inequality P2>P0 is satisfied, it is judged that the detection surface 168 of the workpiece 166 has become seated on the reference surface 165, and such a judgment result is output as an ON signal.
Moreover, since the relationship shown in FIG. 12 exists between the back pressure P2 and the distance X, the position detecting device 150 may determine the distance X corresponding to the back pressure P2, and seating of the workpiece 166 may be judged by comparing the determined distance X with the distance X0.